This invention relates to a thyristor Leonard type elevator control system and more particularly to such a system for controlling an elevator car operated by an emergency AC generator.
Since emergency Ac generators are generally low in capacity, the may be adopted a measure to travel the elevator car operated by an associated emergency AC generator at a speed less than its rated speed determined when the elevator car is operated by a normal AC source therefor. However, with a speed of a hoist DC motor for the elevator car controlled in accordance with a thyristor Leonard system, only a decrease in command speed for the elevator car causes merely a reduction in power factor of the load but does not result in a decrease in load of the emergency AC generator. This is because the thyristor Leonard system controls a phase of an AC voltage applied thereto to adjust the resulting DC voltage. This has resulted in a direct current remaining unchanged though the DC voltage would be changed. Accordingly a corresponding alternating current remains unchanged and the emergency generator does not vary in capacity or a kilovolt-ampere (which is abbreviated to a "KVA"). On the other hand, the emergency AC generator has encountered the problem in a capacity or a KVA of a load therefor.
It has been hereinbefore proposed to decrease the command speed and simultaneously reduce a voltage provided from a source transformer in order to decrease the command speed. A conventional thyristor Leonard type elevator control system has comprised a three-phase AC source transformer receiving selectively an electric power from a three-phase AC source and an emergency three-phase AC generator, a thyristor converter connected to the three-phase source transformer through a pair of three-phase normally open contactors connected to an end of and a tap on the secondary winding of the source transformer, and a firing control circuit connected to the thyristor converter and also a command speed generator.
Further a power failure detecting relay connected to the thre-phase AC source is normally energized and picked up to connect the source transformer to the AC source and disconnect it from the emergency AC generator while the thyristor converter is applied with the entire secondary voltage across the source transformer through that AC connector connected to the end of the secondary winding thereof and put in its closed position. The firing control circuit controls the firing of thyristor disposed in the thyristor converter in response to a command speed signal from the command speed generator. Accordingly the thyristor converter drives an associated hoist DC motor at a speed as determined by the command speed signal resulting in a mating elevator car traveling at its rated speed.
Upon the occurrence of a power failure, the power failure detecting relay drops out to disconnect the source transformer from the AC source and connect it to the emergency AC generator while the thyristor converter receives an intermediate voltage from the source transformer through the remaining AC contactor put in its closed position with the one AC contactor returned back to its open position. At the same time, the command speed generator applies to the firing control circuit another command speed signal less in magnitude than that described above. This is because a set of contacts controlled by the power failure detecting relay is put in its closed position to connect a Zener diode to the output of the command speed generator to permit the Zener diode to clip the command speed signal. As a result, the elevator car travels at a speed less than its rated speed.
Since the source transformer changes in transformation ratio to adjust a DC voltage applied across the armature of the hoist motor as described above, the above-mentioned conventional elevator control system has increased in power factor and the emergency AC generator has decreased in capacity or KVA and been easily operated.
Such a control system, however, has been disadvantageous in that an associated control board is expensive and large-size because a pair of three-phase AC contactors are required to be connected to the source transformer to change a transformation ratio thereof.
When arranged in a thyristor Leonard circuit configuration, the thyristor converter produces a DC output voltage controlled by changing the firing phase of thyristors disposed therein. Therefore an alternating current applied to the thyristor converter lags in phase behind an associated AC voltage resulting in the generation of a high reactive power. As the lower the DC output voltage the higher the reactive power and the lower the power factor thereof will be.
The thyristor Leonard circuit configuration may comprise first three pairs of serially connected thyristors connected together in anti-parallel circuit relationship to second three pairs of serially connected thyristors. Each of the first three thyristor pairs includes the junction of the two thyristor connected to a different one of three-phase conductors subsequently connected to a three-phase AC source as does each of the second three thyristor pairs and the first and second three thyristor pairs are connected across a load, in this case, the hoist DC motor.
Only either one of the first and second thyristor pairs is used and the three-phase AC source supplies a three-phase electric power to the thyristors in the three pairs through the phase conductors while a firing control circuit effects the phase control of those thyristor. Thus a DC output voltage is generated to drive the DC motor.
In the thyristor Leonard circuit configuration as described above, the firing of the thyristors may be controlled in the symmetric or unsymmetric mode. In the symmetric mode the firing angle of those thyristors having anode electrodes connected to the respective phase conductors or the thyristors on the positive side is equal to that of those thyristors having cathode electrodes connected to the respective phase conductors or the thyristor on the negative side with a null DC output voltage. This has resulted in a long duration of a phase current through each thyristor and and therefore an increase in reactive power. In the unsymmetric mode, however, the firing angles of the thyristor on the positive and negative sides may approximate electrical angles of 0.degree. and 180.degree. degrees of the system respectively with a null DC output voltage. Under these circumstances, the two serially connected thyristor in each pair are simultaneously fired for a relatively long time to shortcircuit the DC side. This has resulted in a reduction in duration of a phase current through each thyristor. Thus the reactive power is decreased accordingly.
Therefore the firing control in the unsymmetric mode is advantageous in that the reactive power decreases and the resulting power factor is improved but disadvantageous in that, with the DC output voltages in excess of a certain magnitude, the power factor is not improved and the DC output voltage deteriorates in form factor resulting in an increase in noise generated by the DC motor.
In elevator control systems including the thyristor Leonard type converter it has been already proposed to control the firing of the thyristor converter in the unsymmetric mode with DC output voltages of not larger than a certain magnitude from the thyristor converter and change the control of the firing thereof to the symmetric mode with the DC output voltages in excess of the certain magnitude. As the elevator car is frequently repeated to accelerate and decelerate, the control of the firing of the thyristor converter is frequently changed from the symmetric to the unsymmetric mode and vice versa. This has resulted in the generation of shocks though would be for short times and therefore in the deterioration of a comfortable ride in the car.
However if the firing of the converter is controlled in the unsymmetric mode in the case the elevator car travels at a low speed by an associated emergency AC generator then the resulting power factor can be improved. Further noise from the DC motor may be somewhat increased without hindrance in the operation of the emergency generator because the emergency generator is operated for a short time interval and in case of an emergency.
Accordingly it is an object of the present invention to provide a new and improved thyristor Leonard type elevator control system increased in a power factor at low speed of an elevator car involved by changing the control mode of a firing control circuit for a thyristor Leonard type converter during the travel of the elevator by an associated emergency AC generator.
It is another object of the present invention to provide a thyristor Leonard type elevator control system including the firing control circuit as described in the preceding paragraph manufactured cheaply and free from the deterioration of a comfortable ride in an elevator car involved.